Classification. 2 5 



for instance, has a large number of teeth, and in this 

 feature resembles most fish, while it differs from all 

 mammals. But it also gives suck to its young. Now, 

 looking to these two features alone, should we say 

 that a porpoise ought to be classed as a fish or as a 

 mammal? Assuredly as a mammal ; because the 

 number of teeth is a very variable feature both in fish 

 and mammals, whereas the giving of suck is an in- 

 variable feature among mammals, and occurs nowhere 

 else in the animal kingdom. This, of course, is chosen 

 as a very simple illustration. Were all cases as 

 obvious, there would be but little distinction between 

 natural and artificial systems of classification. But it 

 is because the lines of natural affinity are, as it were, 

 so interwoven throughout the organic world, and 

 because there is, in consequence, so much difficulty in 

 following them, that artificial systems have to be made 

 in the first instance as feelers towards eventual dis- 

 covery of the natural system. In other words, while 

 forming their artificial systems of classification, it has 

 always been the aim of naturalists whether con- 

 sciously or unconsciously to admit as the bases of 

 their systems those characters which, in the then state 

 of their knowledge, seemed most calculated to play an 

 important part in the eventual construction of the 

 natural system. If we were dealing with the history 

 of classification, it would here be interesting to note 

 how the course of it has been marked by gradual 

 change in the principles which naturalists adopted as 

 guides to the selection of characters on which to found 

 their attempts at a natural classification. Some of 

 these changes, indeed, I shall have to mention later 

 on ; but at present what has to be specially noted is, 



